Numerous types of heart valve prostheses have been developed for replacing defective heart valves in human patients. One common type of heart valve prosthesis includes a natural tissue heart valve mounted within a sent. The sent generally provides strength and rigidity to the heart valve. Typically, the sent is covered with a textile material, such as Dacron.TM., which provides a substrate to which the heart valve may be secured. While the sent provides desired rigidity and strength, which inhibits the inward deflection of the sent posts, it also decreases the hemodynamics of the valve. This is because the stent substantially increases the side wall thickness of the prosthesis, which reduces the size of the flow orifice for a prosthesis having a given outer diameter. The textile covering also tends to abrade cusps of the valve.
In order to overcome the disadvantages associated with the stented heart valve prosthesis, there has been an increasing tendency to form natural tissue heart valve prostheses with no stent. These are called stentless valves. Stentless valves exhibit improved hemodynamics and are less resistant to blood flow. In addition, stentless valves, as compared to stented valves, are more resistant to structural failure because the rigidity of a stent can cause damage to the moving cusps. The improved hemodynamic characteristics of stentless valves also can cause beneficial remodeling of the heart muscle. Specifically, it has been determined that several months after implantation of a stentless valve in the aortic position, there is a noticeable improvement in the size of a left ventricle.
Even though a stentless prosthesis offers improved results over its stented counterpart, in practice, conventional stentless prostheses have not been completely satisfactory. It requires a greater degree of surgical proficiency to implant a stentless prosthesis. It usually also requires additional time to perform the procedure. Accordingly, a very small number of surgeons are willing to implant a stentless valve.
There also are technical problems associated with the implantation of a typical natural tissue heart valve prosthesis having no stent. In general, a stentless prosthesis is deformable. Thus, if the aortic annulus is calcified, the implanted valve can be deformed and become dysfunctional. Such deformation of the valve might cause the cusps to be unleveled, resulting in inadequate coaptation of the cusps and backflow. It also has been determined that the sizing of stentless valves is not well defined for surgeons. Therefore, a sizing mismatch may occur, which can cause the valve to be stenotic or insufficient.